Tryptophan metabolism in digestive system tumors: unraveling the pathways and implications

Tryptophan (Trp) metabolism plays a crucial role in influencing the development of digestive system tumors. Dysregulation of Trp and its metabolites has been identified in various digestive system cancers, including esophageal, gastric, liver, colorectal, and pancreatic cancers. Aberrantly expressed Trp metabolites are associated with diverse clinical features in digestive system tumors. Moreover, the levels of these metabolites can serve as prognostic indicators and predictors of recurrence risk in patients with digestive system tumors. Trp metabolites exert their influence on tumor growth and metastasis through multiple mechanisms, including immune evasion, angiogenesis promotion, and drug resistance enhancement. Suppressing the expression of key enzymes in Trp metabolism can reduce the accumulation of these metabolites, effectively impacting their role in the promotion of tumor progression and metastasis. Strategies targeting Trp metabolism through specific enzyme inhibitors or tailored drugs exhibit considerable promise in enhancing therapeutic outcomes for digestive system tumors. In addition, integrating these approaches with immunotherapy holds the potential to further enhance treatment efficacy.


Background
Digestive system tumors rank among the most prevalent malignancies and remain a leading cause of cancer-related mortality, despite advancements in surgery, chemoradiotherapy, and immunotherapy [1][2][3][4].The challenges lie in their elusive early detection and high invasiveness, particularly evident in liver and pancreatic cancers.Addressing this challenge requires focused efforts on identifying precise diagnostic biomarkers and effective therapeutic targets.
Metabolic reprogramming stands out as a hallmark of cancer, altering how tumor cells use energy and nutrients [5][6][7].While normal cells primarily rely on oxidative phosphorylation for energy production, cancer cells favor anaerobic glycolysis.In addition, tumor cells exhibit an augmented demand for biosynthetic pathways, including amino acids, lipids, and nucleotides [8][9][10].Trp metabolism emerges as a novel frontier in tumor metabolism research, given its role as both a component in protein synthesis and a precursor to biologically active molecules [11][12][13].Beyond its physiological regulation in the human body, Trp and its metabolites play a crucial role in tumorigenesis and progression [13].Differentially expressed Trp metabolites serve as potential diagnostic markers for tumors, while tumor cells use Trp to synthesize bioactive molecules influencing cell biological functions through signaling pathways or direct action [14,15].Trp metabolites further impact immune cells and the tumor microenvironment, exerting influence on tumor development [16,17].Investigating Trp metabolism enhances our comprehension of tumor genesis and development, presenting potential targets for innovative therapeutic strategies.
As a vital biological pathway, Trp metabolism significantly contributes to the onset and progression of digestive system tumors.Studies indicate that heightened Trp metabolism in digestive system tumor cells leads metabolite accumulation, affecting tumor growth, metastasis, and immune evasion [18,19].Trp degradation products modulate immune cell function, influencing the immune response to digestive system tumors.In addition, Trp metabolism regulates immune cell function, affecting the immune response to digestive system tumors.In-depth exploration of Trp metabolism has led to novel therapeutic strategies, including inhibiting key enzymes such as indoleamine 2,3-dioxygenase (IDO) and Trp 2,3-dioxygenase (TDO) to block Trp utilization, thereby enhancing immune response and inhibiting tumor growth.Combining IDO1 inhibitors with immune checkpoint inhibitors shows promise in improving treatment efficacy.Ongoing drug development targets the interference of Trp transport and utilization by tumor cells, underscoring the therapeutic potential of Trp metabolism in digestive system tumors.This review comprehensively summarizes the expression of Trp and its metabolites in digestive system tumors, elucidating their correlation with clinical features.In addition, it delves into the roles and mechanisms of Trp metabolism in the development of digestive system tumors.Finally, it introduces therapeutic strategies targeting Trp metabolism, underscoring their potential in advancing the treatment landscape for digestive system tumors.

Overview of Trp metabolism
Trp assumes vital physiological roles within the human body, serving as an essential amino acid for protein synthesis [20].Beyond its structural role, it acts as a precursor to the neurotransmitter serotonin (5-HT), influencing mood regulation, sleep, and appetite control.Trp metabolism encompasses the intricate transformation and utilization of Trp, involving several pathways and key enzymes that regulate crucial physiological functions [21].The three primary metabolic pathways of Trp metabolism are the kynurenine, 5-HT, and indole pathways (Fig. 1).

5-hydroxytryptamine pathway
5-hydroxytryptamine (5-HT, serotonin) is a versatile and ancient bioamine with diverse functions [29,30].Comprising integral components such as the serotonin transporter (SLC6A4), serotonin receptors (HTR), Trp hydroxylase (TPH), and monoamine oxidase, the 5-HT pathway plays a pivotal role in the Trp hydroxylation pathway.The synthesis of 5-HT, a neurotransmitter that regulates emotions, sleep, appetite, and other physiological functions, begins with Trp hydroxylation [29,31].This initial step is catalyzed by TPH, which converts Trp into 5-hydroxytryptophan (5-HTP).The conversion of 5-HTP to 5-HT requires L-aromatic amino acid decarboxylase (AADC) and cofactors like vitamin B6 [32].5-HT binds to various subtypes of 5-HT receptors on targeted cells, including 5-HT1, 5-HT2, and 5-HT3, eliciting diverse physiological effects in different tissues and organs.Monoamine oxidase (MAO) facilitates the metabolism of 5-HT into 5-hydroxyindoleacetic acid (5-HIAA), which is excreted by the kidneys.The 5-HT pathway exerts a crucial regulatory role in tissues and organs such as the digestive system, cardiovascular system, and immune system [33,34].Within the digestive system, 5-HT regulates gastrointestinal peristalsis and secretion.In the cardiovascular system, it participates in the regulation of blood pressure and vasoconstriction.In the immune system, 5-HT plays a role in modulating the function of immune cells.

Indole pathway
The indole pathway constitutes a segment of the Trp decarboxylation (TDC) pathway, which is catalyzed by gut microbes.TDC catalyzes the conversion of Trp into tryptamine, a process mediated by intestinal microorganisms.Tryptamine undergoes further metabolism, generating various indoleacetic acid derivatives, including indoleacrylic acid, indole-3-acetic acid (IAA), and indole-3-acetaldehyde.Indole and its derivatives act as natural ligands for the aryl hydrocarbon receptor (AhR), activating AhR by binding to it and thereby regulating the transcription of downstream genes.Activated AhRligand complexes play a crucial role in regulating diverse biological processes such as detoxification metabolism, immune regulation, and development.AhR promotes the expression of detoxification enzymes, aiding the body in eliminating harmful substances.In addition, AhR regulates the differentiation and function of immune cells, actively participating in immune response regulation.Various substances, including polycyclic aromatic compounds, polychlorinated biphenyls, and Kyn, can influence AhR activity.As a ligand-activated transcription factor, AhR plays a pivotal regulatory role in fundamental biological processes.

Role of Trp mechanism in digestive system tumors Esophageal cancer (EC)
In patients with EC, serum Trp levels are significantly reduced compared to healthy controls, while serum serotonin (5-HT) and 5-HIAA levels are notably elevated in patients with EC compared to healthy controls (Fig. 1) (Table 1) [35,36].Overexpression of IDO2 is observed in ESCC tissues [37,38].Elevated 5-HT emerges as a crucial indicator of EC susceptibility (AUC = 0.811), and increased plasma 5-HIAA is strongly associated with a higher likelihood of lymph node metastasis [36].A higher XA/Kyn ratio is linked to shorter overall survival.Notably, XA/Kyn levels are significantly correlated with lower overall survival in patients with EC [36].Increased TDO2 levels are significantly associated with lymph node metastasis, advanced clinical stage, recurrence status, and unfavorable prognosis in patients with EC [37,38].CD44, a commonly used tumor stem cell marker in EC, can self-renew and rebuild tumor tissues and is notably expressed in ESCCs with high TDO2 expression, indicating its potential role in tumor recurrence, metastasis, and drug resistance [38].Patients with negative IDO1 expression exhibit significantly improved overall survival compared to those with positive IDO1 expression [39].High CD8 expression is associated with better overall survival and lower IDO1 expression.Consistently upregulated ratios of Kyn/Trp, 5HTP/Trp, 5HIAA/Trp, and 5HT/Trp in patients with ESCC emerge as promising biomarkers for tumorigenesis and metastasis [35].Trp and its metabolites show potential as postoperative prognosis markers in patients with EC, with a sharp decrease in plasma total Trp concentration observed after surgery [40].Functionally, TDO2 enhances tumor cell proliferation, migration, and colony formation, with inhibition of TDO2 preventing these effects [37,38].TDO2 overexpression contributes to in vivo tumor growth [37].Mechanistically, TDO2 upregulates interleukin-8 (IL-8), activating the protein kinase B (AKT)/ glycogen synthase kinase-3 (GSK3β) pathway and promoting M2 macrophage polarization, thereby accelerating tumor progression in EC [37].

Gastric cancer
In patients with gastric cancer, serum levels of Trp, melatonin, and indole-3-propionic acid are downregulated, while indole levels are increased [41,42].The expression of TDO2 in gastric cancer tissues and cells is significantly higher than that in the control group (Fig. 1) [43].Trp and its metabolites exhibit associations with various clinical features in gastric cancer.Serum Kyn levels positively correlate with drug resistance in patients with gastric cancer [44].TDO2 expression is positively correlated with T grade, N grade, M grade, and lymphatic invasion status [43].In addition, TDO2 expression is negatively correlated with overall survival and is an independent predictor of gastric cancer prognosis [43].Plasma-free Trp levels demonstrate higher sensitivity in predicting gastric cancer, and Trp levels in gastric juice serve as potential biomarkers for early detection [42,45].Functionally, TDO2 promotes cell proliferation, colony formation, invasive ability, and spheroid formation in gastric cancer.Knockdown of TDO2 significantly decreases the viability of gastric cancer organoids.IDO1 contributes to parietal cell loss and gastric metaplasia, acting as a critical mediator of immune suppression (Fig. 2) [46].IDO inhibits T cell-mediated cytotoxicity and cell proliferation in gastric cancer, playing a crucial role in gastric metaplasia by modulating B-cell activity [46,83].IDO1 and COL12A1 synergistically enhance cell migration through a positive feedback loop mediated by the mitogen-activated protein kinases (MAPK) pathway [47].Kyns derived from gastric cancer cells overstimulate regulatory T cells (Tregs) through the IL-10/ signal transducer and activator of transcription 3 (STAT3)/ B-cell lymphoma 2 (BCL2) signaling pathway, promoting chemoresistance [44].Glutathione peroxidase 2 (GPx2) contributes to the progression and metastasis of gastric cancer by enhancing the KYNU-Kyn-AhR signaling pathway [48].Helicobacter pylori facilitates gastric intestinal metaplasia by activating the kynurenine pathway mediated by KAT2 through the cyclic GMP-AMP synthase (cGAS)-Interferon regulatory factor 3 (IRF3) signaling [49].

Liver cancer
In hepatocellular carcinoma (HCC) tissues, 3-HAA levels are significantly downregulated, while expression of 5-HT1D, KMO, and TDO2 is markedly upregulated (Fig. 1) [50][51][52].TDO2 overexpression correlates closely with HCC tumor size, degree of differentiation, and M phase, with higher TDO2 levels associated with worse overall survival and disease-free survival (DFS) [53].KMO expression is correlated with tumor differentiation, and patients with HCC with elevated KMO expression exhibit decreased overall survival and a higher risk of disease recurrence [52].Elevated 5-HT1D expression is linked to lower overall survival rates and increased recurrence rates in patients with HCC [51].Higher levels of Kyn and Kyn/Trp ratio are associated with an elevated risk of HCC [54].In addition, high IDO1 expression and increased infiltration of CD8 + T cells are significantly associated with overall survival in patients with HCC [55].IDO1 expression serves as an independent prognostic factor for overall survival, DFS, and recurrence-free survival rates in HCC [55,56].kynurenine and kynurenine/ Trp ratio were associated with higher HCC risk.Functionally, TDO2 and KMO stimulate cell proliferation, migration, invasion, and epithelial-to-mesenchymal transition (EMT) in HCC cell lines [52,53,57].Silencing TDO2 in vivo effectively suppresses intrahepatic tumor metastasis of liver cancer cells [57].5-HT1D markedly enhances cell proliferation, EMT, and metastasis in HCC [51].Moreover, 3-HAA inhibits tumor growth and reduces tumor weight in vivo, triggering apoptosis in HCC cells by interacting with YY1 (Fig. 3) [50].TDO2 stimulation facilitates the EMT process by activating the Kyn-AhR pathway [57].5-HT1D promotes HCC progression by activating the PI3K/Akt/FoxO6 signaling pathway [51].circZNF566 promotes HCC progression by sponging miR-4738-3p and regulating TDO2 expression [53].ZNF165 enhances the proliferation and migration of HCC cells by activating the Trp/Kyn/AhR/CYP1A1 axis and substantially enhancing CYP1A1 expression [58].

Application of Trp metabolism in digestive system tumor treatment
Targeting IDO1 and TDO in digestive system tumor treatment IDO1 and TDO, pivotal enzymes in the kynurenine pathway, regulate the generation of various metabolites from Trp.Their involvement in promoting digestive system tumor progression through multiple mechanisms has identified them as potential therapeutic targets.Advances in IDO1 and TDO inhibitors have shown promise in clinical trials.These inhibitors inhibit tumor progression by blocking the activity of IDO1 and TDO through different mechanisms.Notable inhibitors include 1-MT (Table 2), a canonical IDO inhibitor with low toxicity, and its isomer 1-L-MT, which induces mitochondrial damage, reduces CRC cell proliferation, and inhibits CRC development in vivo [67,89].In addition, 1-D-MT inhibits CRC progression; however, its effect is weaker than that of 1-L-MT [89,90].The TDO inhibitor 680C91 induces the G2 phase arrest and apoptosis in CRC cell lines [91].Sertaconazole nitrate dose-dependently inhibits IDO1 expression in CRC cells, promoting autophagy and apoptosis and inhibiting tumor growth [92].Hydroxyamidine inhibitors show efficacy in CRC and pancreatic cancer by targeting IDO1 [93].Hydrogen sulfide downregulates IDO1, increasing CD8 + T cells and inhibiting HCC progression [94].Abrine inactivates the IDO1/JAK1/STAT1 axis, suppressing immune escape and HCC progression [95].Carbidopa, an AhR agonist, inhibits IDO1 expression in PDAC cells, regulating the JAK/STAT pathway to impede pancreatic cancer progression [96].However, unexpected outcomes in IDO1 inhibitor clinical trials may be attributed to AhR activation, impacting immunotherapy [97].Notably, USP14 knockdown upregulates cytotoxic T-cell activity and enhances anti-tumor immunity in CRC by downregulating IDO1 expression without affecting AhR activity, suggesting a potential avenue for CRC-targeted therapy [98].Sodium Tanshinone IIA Sulfonate (STS) inhibits both IDO1 and TDO2, restraining tumor growth and enhancing the anti-tumor activity of PD1 antibodies [99].IDO1/TDO dual inhibitor RY103 blocks the kynurenine pathway in pancreatic cancer, inhibiting cell motility and demonstrating growth inhibition in vivo [100].
Targeting KMO in digestive system tumor treatment KMO, a crucial enzyme in Trp metabolism, represents a rate-limiting step in this pathway and is implicated in the development of various digestive system tumors.Modulating Trp metabolism by inhibiting KMO expression offers a potential avenue for impacting tumor growth and immune response.Slower research progress is observed on KMO inhibitors compared to IDO/TDO inhibitors, possibly due to the complex role and regulatory mechanisms of KMO, requiring further research to understand its specific role in digestive system tumors.However, recent studies underscore KMO as a promising therapeutic target in digestive system tumors.KMO upregulation is significantly observed in liver cancer and CRC, correlating with worsened patient prognosis in these malignancies [52,63].In HCC cell lines, KMO demonstrates a significant enhancing effect on cell proliferation, migration, invasion, and EMT [52].Similarly, KMO aggravates the malignant phenotype of tumors in CRC [63].KMO knockdown shows promise in inhibiting cancer progression by modulating cell biological functions in HCC and CRC.
Targeting TPH in digestive system tumor treatment TPH, a pivotal enzyme in the 5-HT pathway, catalyzes the conversion of Trp to 5-HTP, influencing the synthesis and release of 5-HT.The potential involvement of the 5-HT pathway in digestive system tumor progression highlights TPH as a key therapeutic target.In CRC, 5-HT, through interaction with the HTR3A receptor, promotes tumor progression by activating the NLRP3 inflammasome.In vivo experiments demonstrate that inhibiting TPH1 with 4-chloro-DL-phenylalanine or using the HTR3A antagonist tropisetron reverses this effect [61].In PDAC, 5-HT and HTR2B agonists promote cell proliferation while inhibiting cell apoptosis.The HTR2B antagonist SB204741 impedes tumor growth and enhances prognosis in PDAC [77].In addition, 5-HT knockdown significantly inhibits angiogenesis in CRC [68].Notably, a 5-HT(1D)R antagonist GR127935 suppresses tumor metastasis by regulating the Axin1/βcatenin/MMP-7 axis (Table 2) [101].However, the role of 5-HT in CRC development remains controversial.Selective serotonin reuptake inhibitors decrease CRC risk by enhancing 5-HT expression [105].Kannen et al. found that 5-HT can reduce DNA damage and CRC risk in the early stages of CRC, but its contribution to tumor growth in CRC complicates its overall impact [106].This controversy underscores the need for further research to comprehensively understand the mechanisms of 5-HT and establish a theoretical basis for relevant therapies.

Combining Trp metabolism targeting with other therapies in digestive system tumor treatment
As tumor treatment strategies evolve, including surgical resection, chemotherapy, radiation therapy, targeted therapy, immunotherapy, and hormone therapy, challenges like drug resistance, recurrence, and metastasis persist.Targeting Trp metabolism has gained attention, but its adverse effects on normal physiological processes necessitate further research for safe and effective strategies.Combining targeted Trp metabolism with other therapies emerges as a promising avenue.In liver cancer, PD-1/PD-L1 monoclonal antibody therapy has shown promise by targeting immune checkpoint molecules [95].Combining Abrine and anti-PD-1 antibodies synergistically inhibits HCC tumor growth [95].TDO inhibitors enhance the efficacy of immune checkpoint inhibitors [102].The dual IDO1/TDO2 inhibitor, STS, effectively boosts the anti-tumor effects of the PD1 antibody in CRC [99].Although Phase I clinical trials did not confirm enhanced benefits, further research, possibly focusing on patients with positive IDO tumors, is warranted [97].CAR T cell therapy, an emerging immunotherapy, shows potential in treating CRC [107].
In vivo studies have found that miR-153 enhances CAR T cell immunotherapy by inhibiting IDO1 expression in CRC.Moreover, 1-MT reduces radiation resistance in CRC cell lines, and its combination with radiation significantly inhibits CRC growth compared to monotherapy [103].In HCC, combining rosmarinic acid with IDO1-shRNA demonstrates inhibitory effects on tumor progression in vivo [104].

Conclusion
Trp, an essential amino acid, plays vital roles in physiological processes, serving as a precursor for molecules such as serotonin, melatonin, and niacin (Vitamin B3).Its metabolism is a critical pathway and significantly influences the progression of digestive system tumors.
Interestingly, a study by Ding et al. showed differing views on the role of TDO2 in HCC.They found TDO2 downregulated in HCC tissues and its expression negatively correlated with tumor size, clinical stage, TNM stage, and tumor differentiation[59].Lower TDO2 levels are associated with worse overall survival and DFS in patients with HCC.TDO2 inhibits cell proliferation by inducing cell cycle arrest in vitro and exerts an

Fig. 2
Fig. 2 Molecular mechanisms of Trp metabolism in gastric cancer.Helicobacter pylori infection induced kynurenine pathway activation mediated by kynurenine aminotransferase 2 (KAT2) through cGAS-IRF3 signaling, fostering gastric intestinal metaplasia.IDO1 and COL12A1 exhibited a synergistic effect, enhancing cell migration via the MAPK pathway.GPx2 enhanced the KYNU-Kyn-AhR signaling pathway, promoting gastric cancer progression and metastasis.Kyn generated by gastric cancer cells activated the IL-10/STAT3/BCL2 pathway, excessively stimulating Tregs and crucially contributing to chemoresistance

Fig. 3
Fig. 3 Molecular mechanisms of Trp metabolism in liver cancer.circZNF566 promoted HCC progression by sponging miR-4738-3p and modulating TDO2 expression.ZNF165 enhanced HCC cell proliferation and migration through robust activation of the Trp/Kyn/AhR/CYP1A1 axis, significantly increasing CYP1A1 expression.5-HT1D, through PI3K/Akt/FoxO6 signaling, accelerated HCC progression.TDO2 upregulated p21 and p27 expression, exerting a suppressive impact on cell cycle arrest.The binding of 3-HAA to YY1 induced apoptosis in HCC cells Elevated Trp metabolism leads to the accumulation of Trp metabolites, impacting malignant phenotypes across ESCC, gastric cancer, liver cancer, CRC, and pancreatic cancer.Aberrantly expressed Trp metabolites correlate with clinical features such as lymph node metastasis, TNM stage, risk of relapse, and tumor size in digestive system tumors.Furthermore, these metabolites closely tie to patients' prognosis, showcasing diagnostic potential.Detecting Trp metabolite levels in blood, urine, or tissue samples provides clinicians with valuable insights for early detection, diagnosis, and prognostic assessment.A multi-parameter diagnostic model, integrating Trp metabolite expressions and other clinical indicators, improves diagnostic accuracy, which is crucial for early detection, personalized treatment planning, and treatment response monitoring in digestive system tumors.In digestive system tumors, Trp metabolites contribute to progression, metastasis, and drug resistance through diverse mechanisms.Inhibiting key enzymes in Trp metabolism can reduce metabolite accumulation, impeding their tumor-promoting effects.Adopting a comprehensive treatment strategy involves combining drugs targeting Trp metabolism with immunotherapy, enhancing therapeutic efficacy while mitigating tumor progression, metastasis, and drug resistance.Tailored treatment approaches are essential, considering variations in tumor types and individual circumstances.Despite advancements, the role of Trp metabolism in digestive system tumors requires ongoing research for more effective treatments.

Table 1
Role and mechanism of tryptophan mechanism in digestive system tumors

Table 2
Research on the application of trp metabolism in digestive system tumors